DSL (VDSL) typically works by dividing the frequencies used in a single phone line into two primary "bands". The
ISP data is carried over the high-
frequency band (25 kHz and above) whereas the voice is carried over the lower-frequency band (4 kHz and below). (See the
ADSL article on how the high-frequency band is subdivided.) The user typically installs a
DSL filter on each phone. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies (the human voice). The
DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other.